ProxyGenerator::isShortIdentifierGetter in PHP 7.1 does not support nullable types

ProxyGenerator::isShortIdentifierGetter does not work with PHP7.1 nullable return types.

Does not work
```
public function getId() : ?int
```

Works

```
public function getId() : int
```

If I understood correctly the `ProxyGenerator::PATTERN_MATCH_ID_METHOD` should be updated to include question mark in return type and test cases to have case with nullable return type.